Why do tectonic plates float on the mantle?

Why do tectonic plates float on the mantle?

• A. They are hotter than the mantle
• B. They are less dense than the mantle
• C. They are heavier than the mantle
• D. They are made of gas

Answer: (B)

Buoyancy is what lets tectonic plates float on the mantle. The lithosphere, which includes the crust and the rigid upper part of the mantle, sits on a much hotter, more ductile layer of mantle called the asthenosphere. Because the lithosphere has a lower average density than the mantle material beneath it, it experiences an upward buoyant force that keeps it riding on top. On geological timescales, mantle convection currents in the asthenosphere slowly push and pull the plates, guiding their movement. The idea is simple: something that is less dense than what surrounds it will float, while something denser would sink. Being made of gas isn’t relevant here, and being hotter or heavier would not explain why the plates stay afloat on the mantle.

Buoyancy is what lets tectonic plates float on the mantle. The lithosphere, which includes the crust and the rigid upper part of the mantle, sits on a much hotter, more ductile layer of mantle called the asthenosphere. Because the lithosphere has a lower average density than the mantle material beneath it, it experiences an upward buoyant force that keeps it riding on top. On geological timescales, mantle convection currents in the asthenosphere slowly push and pull the plates, guiding their movement. The idea is simple: something that is less dense than what surrounds it will float, while something denser would sink. Being made of gas isn’t relevant here, and being hotter or heavier would not explain why the plates stay afloat on the mantle.